Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WIFI transceivers, and other communication devices transmit and receive communication signal at various frequencies. For efficient communication, the frequency of the transmit and receive signals is many times higher than the baseband information signal that carries the information to be communicated. Therefore, a transceiver must upconvert the transmit signal and downconvert the receive signal.
Usually, one or more mixers are used to upconvert the transmit signal and downconvert the receive signal. In many radio frequency (RF) communication methodologies, and in a quadrature modulation methodology in particular, a mixer can be implemented using a series of switches that switch differential components of a quadrature signal according to a local oscillator (LO) signal. The frequency of the LO signal is chosen so that a radio frequency signal mixed with the LO signal is converted to a desired frequency.
Signal upconversion and signal downconversion is performed by using mixers, which are typically implemented using semiconductor switches. In deep sub-micron technology the availability of passive switches providing low noise operation and highly efficient operating characteristics enables the use of passive mixers where low current consumption and high performance is desired. Rail to rail voltages used in the switch clock path and issues due to poor isolation between the in-phase (I) and quadrature-phase (Q) paths in the mixer impose limitations on the use of a passive mixer.
A SAW filter is typically used to protect the receive frequency band from interfering signals that may be out of the receive band, but that may still cause interference, particularly at certain multiples (harmonics) of the receive frequency. An LNA is typically used to amplify the relatively weak receive signal so that the information contained therein can be extracted. For a multiband receiver, a separate SAW filter is needed for each band, and a separate LNA is needed to accept the output of each SAW filter. Thus SAW filters and LNAs typically add complexity to the receiver architecture. Further, the LNAs consume power, and this power consumption must be sufficiently high to allow the LNAs to pass large blocking signals without compressing small desired signals.
Therefore, it would be desirable to have a low noise receiver architecture that may not rely on these additional elements.